Hornwort
Ceratophyllum demersum
Also known asCoontail · Rigid hornwort · Coon's tail
Water parameters
Light and nutrients
Substrate type: any. Propagation: fragmentation.
Substrate compatibility
| Substrate | pH effect | Nutrient load |
|---|---|---|
| Bare bottom (no substrate) (Bare bottom) | n/a | none |
| Inert sand (Pool filter sand) | neutral / inert | none |
| Inert gravel (Aquarium gravel) | neutral / inert | none |
| Limestone gravel (Crushed coral) | raises pH | none |
| Aquasoil (ADA Amazonia) | lowers pH | very high |
| Mineralized clay substrate (Seachem Fluorite) | neutral / inert | moderate |
| Dirted tank (mineralized topsoil) (DIY soil substrate) | slightly acidic | very high |
| Wood and rock mounts (Hardscape mount) | varies | none |
With fish
Origin and habitat
One of the most widespread aquatic plants on earth, Ceratophyllum demersum, hornwort or coontail, has a cosmopolitan range across every continent but Antarctica, in ponds, lakes, slow rivers and ditches from subarctic to tropical waters. It is the only living genus of the family Ceratophyllaceae, the sole family of its order. Hornwort is genuinely rootless, lacking roots even as an embryo, and either drifts in the water column or anchors loosely by modified rootless leaves; the stems carry whorls of dark green, forked, needle-like leaves giving a coarse, bristly feel, and can reach 2 m in the wild. It shelters fry, invertebrates and insects in the wild and is sold in the hobby as a fast, near-indestructible oxygenator and one of the most effective nutrient sponges available.
Outdoor pond use
- USDA zones
- 3–13 (winter low around -40°C or warmer)
Care notes
Virtually indestructible. It grows in any light from low to high, across a wide temperature span, in almost any water chemistry and without CO2, and it is best left to float, since it makes no roots; floating it grows fastest and gives great shade and fry cover. Growth is quick, often 3–10 cm a week, fast enough to strip a tank of nutrients, so dose the water column if other plants are competing. The notorious drawback is needle drop: it sheds its fine leaves whenever conditions shift, from temperature swings, low light at the stem base, strong current, nutrient shortfall or liquid-carbon dosing, which can kill it, littering the tank with tiny needles that clog filters. The shedding is usually a passing acclimation phase but turns chronic in poor conditions. Hornwort is also allelopathic, releasing compounds that suppress phytoplankton and blue-green algae while it soaks up nitrogen and phosphorus, which helps with algae but can nudge sensitive neighbours. Propagate by snapping off any piece with a few whorls. In aquaponics it is a tough, effective floating nutrient filter; trim it hard and often.